Protective helmets and armor have been used for both military and non-military applications to provide personnel protection, and are designed through a number of mechanisms, including elastic-plastic deformation, strain delocalization and shock impedance mismatch. These ballistic-resistant articles are typically formed by combining multi-layers of high strength fibers consolidated in a polymer matrix material. Thermoset resins are commonly used in a fiber reinforced composite structure for ballistic resistance. These include, but are not limited to, butyl rubber toughened phenolics along with aramid fabric reinforcements.
Also, thermoplastic resins are used in both rigid and flexible composites applications. These include elastomers such as styrene-isoprene-styrene and styrene-ethylene/butylene-styrene block copolymers, as well as polyurethanes. One of the major advantages of these thermoplastic resins is their applicability in the form of aqueous dispersions without the need of organic solvents, thus lessening environmental concerns. These types of thermoplastic resins are often used in conjunction with polyethylene fibers or the hybrid of aramid and polyethylene fibers to achieve the desired ballistic impact performance and stiffness requirements.
While conventional helmets and armor are suitable for protection against small arms fire (e.g., 9 mm hand-gun bullets) and fragmentation, these designs are not capable to divert and dissipate shockwaves from the head and thus are unable to prevent subsequent traumatic brain injury. Protection against blast and shockwaves, however, requires substantial improvement over such helmets due to the significantly increased pressure and energy expelled by the blast and shockwaves. In fact, it has been reported that soldiers in Iraq and Afghanistan when encountered shockwaves as a result of air blasts with overpressures in the kPa-1 MPa range often suffered mild to severe traumatic brain injury (TBI), which is detrimental and can result in long-term medical problems.
Thus, further improvements in protection against blast and shockwaves would be beneficial.